Usually, a bone screw and in particular a locking screw for fixating a bone nail is inserted into a bone by opening the tissue surrounding the bone, to achieve an access to the bone at a site at which it is intended to introduce a screw. Before screwing in the screw, a bore is drilled into the bone forming a channel for the screw. Into such a channel, a bone screw may be screwed in.
A critical aspect of such an insertion of a bone screw is that the screw should have a length which is long enough to provide sufficient stability, i.e. as much as possible of the thread of the screw should engage with hard bone tissue (Corticalis), but the screw should, on the other hand, have a length which is not too long so that at least one end section of the screw may protrude out of the bone which may cause irritations and insurances at the surrounding soft tissue, i.e. muscles or sinews. Having in mind that a physician will usually have access to a bone only from one side, i.e. at the entry point for the screw, it is difficult to measure the length or depth of a bore into which a screw should be introduced.